Make PyGreSQL work with Python 3.x

PyGreSQL so far only works with Python 2.x, we should make it work with Python 3.x, too.

This has been discussed on the mailing list already (e.g. ​here and ​here) and we decided to create a common codebase for Py 2.x and 3.x starting with Pygres 5.0. We can use 2to3 and preprocessor macros to adapt the common codebase to Python 3.